This invention relates to an apparatus for checking patterns formed on printed circuit boards for use in an electronic apparatus and more particularly to a pattern checking apparatus for checking geometrical defects of a two-dimensional pattern by imaging the two-dimensional patterns and a two-dimensional reference pattern and comparing them with each other.
In recent years, high density packaging of the printed circuit board for use in electronic apparatus has drastically been advanced and consequently, development has been directed to enlargement of the printed circuit board and its formation into a multi-layer structure with fine patterns. Typically, a pattern formed on a printed circuit board is checked by imaging the pattern and a reference pattern formed on another circuit board and comparing them with each other. Because of fineness of these patterns, it is necessary that one printed circuit board carrying the reference pattern and the other to be checked for its pattern be accurately be held in predetermined positions and those patterns for comparison be imaged simultaneously. Conventionally, this type of checking apparatus based on pattern comparison has an inspection table provided, at predetermined positions, with guide pins, which are fitted in holes formed in the printed circuit boards to complete mounting of these boards to the inspection table.
By using these guide pins, the printed circuit boards can be positioned with a displacement of approximately 0.1 to 0.2 mm. Conventionally, since the printed circuit board has patterns of a relatively large width and the slight displacement in positioning can fall within a tolerance of the pattern width, checking can be accomplished with sufficient accuracies by positioning the boards through the use of guide pins. However, a printed circuit board, enlarged with an attendant decrease in thickness of its substrate, tends to have a dimensional error of about 0.1% which is due to such factors as working distortion caused by printed circuit board fabrication processes and thermal contraction attributable to temperature changes. In addition, because of fineness of patterns, a printed circuit board carrying patterns of a pattern width of about 0.1 mm has initially been practiced. For these reasons, a need of confining the accuracy for positioning within several of tens of microns arises and the conventional positioning based on only the guide pins can not meet the need. Further, conventionally, the two printed circuit boards to be compared to each other are positioned by manually adjusting images displayed on a CRT screen so as to match them and therefore, accurate positioning operations are very difficult to achieve.